(i) Field of the Invention
The present invention relates to a self-discharge type pulse charging electrostatic precipitator and more particularly, to the electrostatic precipitator (hereinafter abbreviated as EP) applied to increase precipitation efficiency by suppressing back back ionization and realizing a cost reduction of the power source.
(ii) Prior Art Statement
A conventional EP adopts a negative direct current (DC) high voltage charging method. With this conventional EP, when high-resistive dust is processed, dielectric breakdown is caused in the dust layer on the electrode for precipitation and ions having the reverse polarity are produced, so that the precipitation efficiency is remarkably deteriorated, that is, the back ionization phenomenon occurs. The back ionization phenomenon is caused when the product .rho.d .times.i of the electrical resistivity .rho.d of the dust and the current density i of the dust layer exceeds the dielectric breakdown voltage Edc of the dust layer.
Thus, a pulse charging system is proposed as means for obtaining high precipitation efficiency while suppressing the back ionization.
FIGS. 4(A) and (B) show an example of a pulse superposition type charging EP in which a pulse voltage is superposed on a high DC voltage, and FIGS. 5(A) and (B) show voltage waveforms of the circuit of FIGS. 4(A) and (B). A voltage stepped up by a transformer 1 is rectified through a rectifier 2 and is stored as the electrical charge in a charging capacitor 3. The circuit of FIG. 4 produces the LC resonance by a resonance circuit consisting of the charging capacitor 3, a coupling capacitor 6, a capacitance C.sub.EP contained in an EP 7 and an inductance of the circuit when a high-speed switching element 4 is turned on, and the electrical charge stored in the capacitor 3 is subjected to the LC resonance so that a high voltage having a sharp rising edge is supplied to the EP 7. The switching element 4 is turned off at the next instance, and at this time the charge remaining in the circuit is removed through a waveform shaping resistor 5 so that excessive current does not flow in the EP 7 due to the charge stored in the circuit. In this manner, a voltage having a sharp rising edge and short pulse width can be impressed on the EP 7 through the coupling capacitor 6 as shown in FIGS. 5(A) and (B). Further, in order to obtain a base voltage existing at the time except the occurrence of the pulse, a high DC voltage generator 8 is connected to the EP 7. With this method, the DC charging portion can impress the high peak voltage to the EP without increased average current at the pulse portion while suppressing current and hence the precipitation efficiency for high resistive dust is improved.
However, the above system requires two power sources and a coupling capacitor in addition to the charging capacitor, and hence the cost of the power source is very expensive. Accordingly, the system is not widely put to practical use.
An energy withdrawal type pulse charging system is proposed as another system. However, the system has a complicated power supply circuit and the cost of the power source is expensive.
Accordingly, there has been proposed a self-discharge type pulse charging EP as shown in FIGS. 6(A) and (B) in which the charging capacitor 3 is directly connected to the EP 7 through the high-speed switching element 4 with the coupling capacitor 6, the DC high voltage generator 8 and the waveform shaping resistor 5 being removed. FIG. 6B shows an electrical equivalent circuit of EP which consists of a parallel circuit of an equivalent capacitance C.sub.EP and an equivalent resistance R.sub.EP. With this EP, when the switching element 4 is turned on, the electrical charge stored in an equivalent capacitance C.sub.EP of the EP 7 is discharged through an equivalent resistor R.sub.EP (resistance and the like by the corona discharge) in the EP. FIGS. 7(A) and (B) show voltage waveforms obtained from the circuits of FIGS. 6(A) and (B). The system is characterized in that a pulse voltage waveform having a sharp rising edge can be obtained economically and the uniform current density in the same manner as the prior art pulse charging system can be also obtained, and it has been confirmed by an experiment that the precipitation efficiency for the high resistive dust is improved as compared with the DC charging system.
In FIG. 6(A), a voltage stepped up by the transformer 1 is rectified through the rectifier 2 and is stored as the electrical charge in the charging capacitor 3. Thus, when the high-speed switching element 4 is turned on, the circuit of FIG. 6(A) produces the LC resonance by a resonance circuit consisting of the equivalent capacitance C.sub.EP of the EP 7, the charging capacitor 3 and the inductance of the circuit, and the electrical charge stored in the capacitor 3 is subjected to the LC resonance so that a high voltage waveform having high rising edge as shown in FIGS. 7(A) and (B) is obtained. After the switching element 4 is turned off, the electrical charge stored in the equivalent capacitance C.sub.EP of EP 7 is discharged through the equivalent resistance R.sub.EP of EP 7 and the voltage on the capacitance of EP 7 is gradually attenuated until the switching element 4 is turned on again. In this operation, a starting voltage when the attenuation of the voltage starts by the flow of current through EP 7 after the switching element 4 is turned off is named an attenuation starting voltage, and a lowest voltage just before the switching element 4 is turned on is named a residual voltage.
However, the conventional self-discharge type pulse charging system which is inexpensive has the following problems.
(1) Since the self-discharge type pulse charging system has only a single power source, if a peak voltage is increased in order to improve the efficiency thereof, the attenuation starting voltage and the residual voltage are also increased uniquely. Accordingly, current flowing through the EP is increased while the voltage is attenuated from the attenuation starting voltage to the residual voltage, and hence back ionization is caused for high resistive dust. In particular, since the current flowing through EP is increased in the manner of an exponential function with the increase of the voltage, large current flows in the vicinity of the attenuation starting voltage, thereby producing a critical condition of back ionization. FIG. 8 shows the relationship between the peak voltage and the precipitation efficiency obtained from an experiment made by the inventors. According to the experiment, it has been confirmed that the precipitation efficiency increases with the increase of the peak voltage and has a maximum valaue at a certain peak voltage, the efficiency being reduced above the certain peak voltage.
(2) While the precipitation efficiency is improved as the average voltage from the attenuation starting voltage to the residual voltage, as shown in FIGS. 7(A) and (B), (which corresponds to the average voltage of the so-called DC charging system in the prior art) is higher, it is necessary to shorten the cycle of the pulse shown in FIG. 7(A) in order to increase the average voltage at the period described above. However, in this case, if the cycle of the pulse is shortened excessively, the increase of the current flowing through the EP as corona current in the vicinity of the attenuation starting voltage causes back ionization.
(3) To the contrary, if the cycle of the pulse is enlarged to reduce the consumption energy, the average voltage is reduced.
(4) Further, the self-discharge type pulse charging system can reduce the cost greatly as compared with the conventional pulse charging system. However, in order to increase the capacity of the EP 7 for one power source, the charging capacitor having a large capacitance is required since the charging capacitor 3 is proportional to the equivalent capacitance C.sub.EP of the EP 7. Further, there are technical problems such as the voltage having a round rising edge by increased current flowing through the high-speed switching element and increased inductance contained in the circuit. To the contrary, if the capacity of the EP 7 for one power source is reduced, the economical efficiency is deteriorated since the number of the power sources is increased.